The term “dispersion” is understood to relate to a multi-phase system, which at least consists of components that are essentially not soluble in one another. Dispersions comprise in particular emulsions, in which one liquid is distributed in the form of droplets in another liquid. The phase forming the droplets is referred to as the disperse phase or inner phase. The phase in which the droplets are distributed is referred to as the continuous phase or outer phase.
Dispersions furthermore comprise suspensions in which solid particles are dispersed in a liquid continuous phase. Material systems in which both solid and liquid phases are present in a dispersed form are also counted among dispersions. A solid could, for instance, be present in a distributed form in a first liquid, while this suspension forms the disperse phase of an emulsion. Solids can also be distributed in the continuous phase of emulsions. These may in this context also be referred to as suspo-emulsions.
If two liquids that are essentially not soluble in one another are mixed with each other so that both phases are accessible, material system thus produced is referred to as a mixture. A mixture can be diluted by adding either the one or the other phase. In an emulsion, the disperse phase is, by contrast, not accessible from outside; an emulsion can only be diluted by adding the continuous phase. In producing an emulsion, a mixture can occur as an intermediate stage.
The term “component” will be used herein below to describe in particular one phase of a dispersion. A component may, however, also be a constituent of a phase. A phase can, for instance, be formed by several components that are in particular soluble in one another.
When producing dispersions in particular in the production of emulsions, it is important that the steps required for introducing the inner phase into the outer phase for the production of a premix, and for fine dispersion and stabilization of the product thus obtained, are reliably performed in a defined process, in order to produce a final product with the intended characteristics respecting size distribution of the disperse phase, and flow properties and stability of the product in the presence of thermal and mechanical loads and changes over time. A similar process known from private cooking practice is the production of mayonnaise. The oil phase is gradually stirred into the water phase. This first of all generates a coarse and low-viscosity emulsion as a premix. Continued and quick stirring then produces a finer emulsion, and the viscosity increases. A number of different processes are available for industrial-scale production of dispersions, and in particular emulsions. Which of these processes is used depends on the type of dispersion, and on the fineness of the disperse phase, which can generate a dispersion that is stable for the required period of time. A stable dispersion is understood to be a material system, in which the particle size distribution of the disperse phase and/or the flow properties of which, in particular the viscosity of which, do not change in any essential manner during the defined period.
For the industrial-scale production of dispersions, vessels equipped with a stirrer, for instance a scraper stirrer or a stirrer turbine, are often used for relatively coarse dispersions. For finer dispersions, two-stage processes are used, in which first a premix is produced in a stirred tank, after which the premix is passed through a rotor-stator dispersing machine. This machine could, for instance, be a colloid mill. Very fine dispersions can be produced with a dispersion process in a high-pressure homogenizer as an additional step.
When using a premix having been mixed in a stirred vessel for the production of a fine dispersion in a rotor-stator system, the dispersion is usually assumed to have a very wide particle size distribution. The example to be considered here is an emulsion with a droplet size distribution between 30 and 500 μm. In a conventional rotor-stator system (cf. FIG. 11; see description below), the droplets of the premix, which in the case of an emulsion may also be referred to as a raw emulsion, are reduced in size until a mean droplet size has been reached that corresponds to the specific energy input of the rotor-stator system (energy density). For a relatively narrow droplet size distribution with droplet diameters between 5 and 10 μm and below, it will normally be necessary to run several rotor-stator system passes. Often as many as 5 to 10 passes are necessary. This, on the one hand, exposes the product to considerable mechanical stresses, while the high thermal input, on the other hand, does not permit the energy supplied to be utilized efficiently.
In order to accelerate the process described above, some manufacturers of dispersing machines have started to apply the inner phase directly in front of the rotor teeth or to the rotor teeth of a rotor-stator system, using such means as pipes or boreholes. These rotor-stator systems are described in WO 00/01474 and U.S. Pat. No. 5,590,961. In these systems, the inner phase enters the rotor-stator system and its dispersion zone within a rather limited region and hence in a relatively compact jet. During the first pass through the rotor-stator system, an emulsion is thus produced that is characterized by a wide droplet size distribution, because the inner phase cannot be sufficiently finely blended into the outer phase, which is due to the fact that the spatial limitation does not make a sufficiently large exchange surface available for contact. The droplets in the emulsion, in addition, tend to coalesce, because many small droplets are formed within a small volume so that they cannot be separated and stabilized quickly enough. Another consequence observed in this context is the formation of schlieren. The larger the volume of inner phase added, the more distinct are the coalescence and schlieren effects. In this way, small amounts of the inner phase can be fed into the outer phase. When larger amounts of the inner phase have to be applied, this can produce considerable problems. These problems result from the fact that it is not possible to produce a homogeneous raw emulsion, or a homogeneous premix, with a definable particle size distribution from the outer and the inner phases, before the phases enter the zones of high shear forces in the rotor-stator system.
WO 01/56687 (PCT/EP00/117700) describes a rotor-stator system whose rotor comprises a premixing chamber. The premixing chamber opens into several small chambers on the circumference of the rotor. All the chambers together act as one premixing chamber in the rotor, which is accommodated in the dispersion compartment and which rotates when the rotor-stator system is in operation. Because of the rotor geometry and the volume which is thus available as a premixing chamber, the amount of the inner phase that can be introduced into the outer phase is rather limited. Since the premixing chamber is located in the rotor, and thus in a section of the rotor-stator system which is in motion, the production of dispersions with complex composition and different components, some of which have to be simultaneously introduced into an existing mixture, becomes a very complicated, if not impossible, process.